High performance scenarios in the HL-2A tokamak are studied by numerical modelling. Through shifting the plasma column outwards, a shaped plasma with significant triangularity is achieved with sufficient room left for the RF antenna. For the out-shifted, shaped plasma, ripple loss of high energy ions during neutral beam injection (NBI) is analysed, and the results show that the ripple loss fraction of NBI power for the shaped plasma is no higher than that for the unshifted circular plasma. The time dependent TRANSP code is used to model realistic reversed magnetic shear (RS) operation in such plasmas. In order to sustain the RS operation towards steady-state, an off-axis current drive with a lower hybrid wave at 2.45 GHz is used to control the current profile. A steady-state RS discharge is formed and sustained until the LH power is turned off; the plasma confinement is enhanced with the development of an internal transport barrier. In the RS discharges with shaped plasma geometry, a double transport barrier is developed. To understand the underlying physics for the current profile control with LHCD in HL-2A, the LH wave deposition in plasmas with RS is analysed.
Physics related to fast electrons in lower hybrid (LH) current drive (LHCD) plasma is a very important issue, since these particles will play an important role in runaway electron (RE) generation and lower hybrid wave (LHW)-related physics. Utilizing a new hard X-ray (HXR) pinhole camera, recent HL-2A tokamak experiments have devoted to enhancing the understanding of the physics on fast electrons and LHW. The fast electron bremsstrahlung (FEB) emission in the HXR energy range between 20 and 200 keV was measured by the HXR camera. To study the conversion of LHW-produced fast electrons into REs, a very short pulse of LHW, so-called “blip”, with duration of 5 ms was injected into the plasma during the current flattop phase. A strong enhancement of REs was induced by the blip injection. Measurements from the HXR camera show that the fast electrons generated by LHWs is mainly concentrated in 40-60 keV, which is well consistent with the calculated value based on Landau damping theory. The energy of these seed electrons is higher than the critical runaway energy. This phenomenon may be come from the synergetic effects of Dreicer and avalanche RE generation. Moreover, the measurements indicate that the spatial distribution of the fast electrons during LHCD has a peaked profile, implying that the fast electrons are mainly produced in the plasma core. It also suggests that the energy of the LHW mainly deposited in the plasma core region.
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